Balance shafts which are utilized to offset the cyclic shaking forces of rotating and reciprocating engine masses are required to maintain substantially fixed angular timing relationships with the engine's crankshaft. Chain drives and gearsets are both capable of this functionality, but both introduce acoustic emission issues when trying to do the job alone. Toothed belt drives are feasible but are generally unsuited to application requirements.
“Chain Alone” Challenges—Chain drive systems having the automatic tensioning devices customarily needed to accommodate a lifetime of component wear can quite comfortably manage the operating center distance (hereafter “center distance”) variations that challenge gearsets, but carry acoustic emission issues of their own. Inherent to any chain drive system is the polygonal motion or so-called chordal action of the segmented chain's engagement with its sprockets, which is exaggerated in the case of smaller, lower toothcount sprockets. Meshing excitations become more severe with the square of increases in chain velocity, as the radial displacements and tangential velocity variations of chordal action become compressed into ever-tighter timeframes. A single-stage 2:1 step-up ratio balance shaft chain drive system, with its 2:1 difference in sprocket sizes, is acoustically challenged by the high chordal action of its relatively “undersized” driven sprocket being combined with the high chain velocities associated with its much larger driver (crankshaft) sprocket. The chain meshing forces excite engine structures, often resulting in audible emissions.
“Gearset Alone” Challenges—In the case of direct drive gearsets connecting a balance shaft apparatus with a crankshaft-mounted drive gear, the principal engineering challenge is the management of the substantial variations in center distance imposed on the gears by differential thermal expansion effects, tolerance stack-ups, and crankshaft mobility. The result of center distance variation between gears is variation in the backlash, or operating clearance, between mating teeth.
Insufficient backlash (forced tight mesh) results in greatly increased meshing noise (or “whine”), and risk of tooth fatigue due to the large cantilever bending loads imposed by the wedging together of the teeth in mesh. Excessive backlash magnitudes allow sufficient tooth separation magnitude, under the crankshaft's ubiquitous torsional accelerations, as to result in tooth closure impact energy that is large enough to overcome oil film cushioning effects, with the unpleasant result being acoustic emissions (or “rattle”).
Oil film cushioning effects are maximized with gear geometry and operating alignment controls that ensure high values of effective total contact ratio (the actual average number of teeth in contact, hereafter “contact ratio”). With current low viscosity oils and elevated operating temperatures, however, the tooth closure energy associated with excessive backlash can overwhelm the energy absorption capabilities of optimized oil film cushioning effects. The center distance variations associated with contemporary engine thermal effects alone are so large as to incur backlash variations which compromise the acoustic performance of conventional direct drive gearsets under very ordinary thermal operating ranges.
Scissors gears and so-called Vernier gears have been utilized for anti-lash drive systems in cases of relatively low mesh velocity where packaging space and cost constraints permit, but the crankshafts of contemporary high speed gasoline engines are not among these cases. The drawbacks of scissors gears are known to include meshing noise, durability, and very high manufacturing cost. Meshing noise arises from the high tooth loadings which accompany the resilient biasing between side-by-side paired (or “split”) gear members, and is exacerbated by the compromises in contact ratio that result from the packaging space sharing that is required of these side-by-side gear members. Durability challenges are posed by the abnormally high tangential tooth loading required to directly convey the inertia torques of torsional vibrations imposed on the gearsets, in conjunction with the packaging space-dictated narrowness of gear members. Substantial manufacturing costs arise from the extreme precision required for location and runout control of the biasing gear member with respect to the fixed member, and the high material property demands placed by the high tangential tooth loads.
Accordingly, need exists for practical and cost effective inventive methods and structures for the control of the backlash of crankshaft to balance shaft apparatus gearsets over a wide range of operating temperatures, without invoking the noise, durability, and manufacturing cost compromises associated with the complexity, tooth loading, and packaging space sharing that scissors and vernier gear drives comprise.